1. Field of the Invention
The present invention relates to water directing apparatus for changing the streamlines of water flowing across the planing surface of a jet powered watercraft into the inlet of the jet pump inlet passage with minimum drag and flow separation.
b 2. Description of the Prior Art
Jet powered watercraft are driven by a pump impeller discharging water through a rearwardly directed discharge nozzle. The impeller draws water from a forwardly and downwardly extending pump inlet passage which terminates in an inlet located in a flat portion of the watercraft planing surface. The planing surface may be in the hull in certain types of watercraft, or located in a ski in other designs. In either event, the pump inlet passage is relatively steep to conserve space in the watercraft and a combination of pump suction and ram effect is relied upon to bring water to the impeller.
Water is drawn into the inlet passage primarily through pump suction on start-up and at low speeds. As watercraft speed increases water flow in the inlet passage becomes turbulent and flow separation occurs, particularly in the sloping upper forward portion of the inlet passage. This has an adverse effect on the efficiency and overall performance of the jet pump system.
A related problem is the tendency of the watercraft planing surface to bounce out of the water during rough water conditions. Since the jet pump is essentially a constant power device, when the planing surface separates from the water, air is ingested, cavitation occurs, and pump suction and jet drive are lost. The jet drive abruptly resumes when the planing surface drops to the water. This full onfull off action unduly stresses the structure and gives a poor ride, effects which are even more pronounced in smaller watercraft such as the "WETBIKE" watercraft described in U.S. Pat. No. 3,948,206, issued Apr. 6, 1976 to Nelson Tyler.
One solution to the problem of achieving smooth redirection of water into a jet pump inlet passage is disclosed in U.S. Pat. No. 3,757,728, issued Sept. 11, 1973 to Ralph A. Rhoda. Rhoda provides a guide vane whose leading edge he requires to be located outwardly and a substantial distance forwardly of the inlet of the pump inlet passage. He further requires that the trailing or downstream edge of the guide vane be located a substantial distance deep within the pump inlet passage. The concept is apparently to extend the guide vane sufficiently into the inlet passage that water flow is divided into upper and lower flow portions in a manner analogous to the way in which a pipe elbow changes the direction of fluid flow. In effect, the pipe inner surfaces extend from a point well forward of the inlet passage and empty deep within the inlet passage.
U.S. Pat. No. 4,237,812, issued Dec. 9, 1980 to Jerald S. Richardson discloses yet another means to facilitate induction of water into the inlet of a jet pump, in this case a jet pump for a jet propelled ski craft. Richardson provides a grating for attachment across the pump inlet. The single opening in the grating has a rearward terminus in the form of a single concave scoop located immediately adjacent the downstream edge of the pump inlet.
The water directing systems of these patents and others of the prior art are either ineffective to direct the boundary layer water flow into the inlet without turbulence and flow separation, or they are relatively large and complex, and extend deep within the pump inlet passage to define internal conduits or pipes to turn the water flow.